1. Field of the Invention
The present invention generally relates to the field of semiconductor devices and methods of fabricating the same. The present invention particularly relates to semiconductor diodes and their methods of fabrication.
2. Prior Art
Semiconductor devices of various kinds are well known in the prior art. The present invention relates to methods of fabricating semiconductor diodes and the diodes so fabricated, though the methods are also directly applicable to the fabrication of transistors as well. Because of the emphasis on diodes herein, only the prior art relating thereto will be discussed.
Semiconductor diodes are widely used in electronic circuits for various purposes. The primary purpose of such semiconductor diodes is to provide conduction of current in a forward direction in response to a forward voltage bias, and to block conduction of current in the reverse direction in response to a reverse voltage bias. This rectifying function is widely used in such circuits as power supplies of various kinds as well as in many other electronic circuits.
In typical semiconductor diodes, conduction in the forward direction is limited to leakage current values until the forward voltage bias reaches a characteristic value for the particular type of semiconductor device. By way of example, silicon pn junction diodes don""t conduct significantly until the forward bias voltage is at least approximately 0.7 volts. Many silicon Schottky diodes, because of the characteristics of the Schottky barrier, can begin to conduct at lower voltages, such as 0.4 volts. Germanium pn junction diodes have a forward conduction voltage drop of approximately 0.3 volts at room temperature. However, the same are currently only rarely used, not only because of their incompatibility with silicon integrated circuit fabrication, but also even as a discrete device because of temperature sensitivity and other undesirable characteristics thereof.
In some applications, diodes are used not for their rectifying characteristics, but rather to be always forward biased so as to provide their characteristic forward conduction voltage drop. For instance, in integrated circuits, diodes or diode connected transistors are frequently used to provide a forward conduction voltage drop substantially equal to the base-emitter voltage of another transistor in the circuit. While certain embodiments of the present invention may find use in circuits of this general kind, such use is not a primary objective thereof.
In circuits which utilize the true rectifying characteristics of semiconductor diodes, the forward conduction voltage drop of the diode is usually a substantial disadvantage. By way of specific example, in a DC to DC step-down converter, a transformer is typically used wherein a semiconductor switch controlled by an appropriate controller is used to periodically connect and disconnect the primary of the transformer with a DC power source. The secondary voltage is connected to a converter output, either through a diode for its rectifying characteristics, or through another semiconductor switch. The controller varies either the duty cycle or the frequency of the primary connection to the power source as required to maintain the desired output voltage. If a semiconductor switch is used to connect the secondary to the output, the operation of this second switch is also controlled by the controller.
Use of a semiconductor switch to couple the secondary to the output has the advantage of a very low forward conduction voltage drop, though has the disadvantage of requiring careful control throughout the operating temperature range of the converter to maintain the efficiency of the energy transfer from primary to secondary. The use of a semiconductor diode for this purpose has the advantage of eliminating the need for control of a secondary switch, but has the disadvantage of imposing the forward conduction voltage drop of the semiconductor diode on the secondary circuit. This has at least two very substantial disadvantages. First, the forward conduction voltage drop of the semiconductor diode device can substantially reduce the efficiency of the converter. For instance, newer integrated circuits commonly used in computer systems are designed to operate using lower power supply voltages, such as 3.3 volts, 3 volts and 2.7 volts. In the case of a 3 volt power supply, the imposition of a 0.7 volt series voltage drop means that the converter is in effect operating into a 3.7 volt load, thereby limiting the efficiency of the converter to 81%, even before other circuit losses are considered.
Second, the efficiency loss described above represents a power loss in the diode, resulting in the heating thereof. This limits the power conversion capability of an integrated circuit converter, and in many applications requires the use of a discrete diode of adequate size, increasing the overall circuit size and cost.
Another commonly used circuit for AC to DC conversion is the full wave bridge rectifier usually coupled to the secondary winding of a transformer having the primary thereof driven by the AC power source. Here two diode voltage drops are imposed on the peak DC output, making the circuit particularly inefficient using conventional diodes, and increasing the heat generation of the circuit requiring dissipation through large discrete devices, heat dissipating structures, etc. depending on the DC power to be provided.
Therefore, it would be highly advantageous to have a semiconductor diode having a low forward conduction voltage drop for use as a rectifying element in circuits wherein the diode will be subjected to both forward and reverse bias voltages from time to time. While such a diode may find many applications in discreet form, it would be further desirable for such a diode to be compatible with integrated circuit fabrication techniques so that the same could be realized in integrated circuit form as part of a much larger integrated circuit. Further, while reverse current leakage is always undesirable and normally must be made up by additional forward conduction current, thereby decreasing circuit efficiency, reverse current leakage can have other and more substantial deleterious affects on some circuits. Accordingly it would also be desirable for such a semiconductor diode to further have a low reverse bias leakage current.
In many applications it is required that the diode be put across a coil such as a transformer. In these instances it is possible for a reverse voltage to be applied to the diode which will force it into reverse breakdown, specifically into a junction avalanche condition. This is particularly true in DC to DC converters which use a rapidly changing waveform to drive transformer coils which are connected across diode bridges. In these applications a specification requirement for xe2x80x9cAvalanche Energyxe2x80x9d capability is a parameter normally included in the data sheets. The avalanche energy capability of a diode is a significant factor for a designer of such circuits. The avalanche energy capability determines how much design margin a designer has when designing a semiconductor diode into a circuit. The larger the number of avalanche energy capability the more design flexibility a circuit designer has.
The avalanche energy capability is a measure of the diode""s capability to absorb the energy from the coil, where energy E=(xc2xd)*I2*L, without destroying the diode. These requirements are typically on the order of tens of millijoules. A key factor in the ability of a diode to nondestructively dissipate this energy is the amount of junction area which dissipates the energy i.e., the area of the junction which actually conducts during avalanche. It is desirable to increase the avalanche energy capability of a semiconductor diode to improve its utilization.
The present invention includes methods and apparatus as described in the claims. Briefly, semiconductor diodes having a low forward conduction voltage drop, a low reverse leakage current, a high voltage capability and enhanced avalanche energy capability, suitable for use in integrated circuits as well as for discrete devices are disclosed. The semiconductor diodes are diode connected cylindrical field effect transistors having one diode terminal as the common connection between the gate and the drain of the cylindrical field effect transistors. The method of processing the field effect transistor provides very short channels, shallow diffused regions and trench terminated junctions at the edges of the active device. The trench terminated junctions are formed out of a vertical etch cut through the P-N junction at the edge of the device forming the trench which is then passivated with a dielectric material to provide a region of higher breakdown voltage at the edge of the device than is seen within the active device area. The trench terminated junction results in spreading the breakdown energy over the entire active device region rather than just device edges. The preferred fabrication technique for the active device uses two masks and two masking steps, without any critical mask alignment requirements. Various embodiments are disclosed.